1. Field of the Invention
The present invention relates to a turbine, such as a gas turbine or a steam turbine, and a turbine rotor blade for the turbine.
2. Description of the Related Art
Turbine rotor blades of gas turbines and steam turbines are continuously excited for vibrations of frequencies in a wide frequency range by turbulent components of a working fluid. The vibratory response of a blade structure to excitation is influenced by the magnitude of excitation and damping for natural free vibration frequency in each mode of vibration. To improve the reliability of blades, a blade connecting structure is employed to connect the adjacent blades so that resonance may be avoided in a lower order vibration mode in which vibration response, in generally, is high and vibration response may be low in a higher degree vibration mode in which vibration response is low even if resonance occurs.
A blade connecting structure includes connecting covers, namely, integral covers, attached to the outer edges of blade profile parts so as to extend in the revolving direction of the blades so that the integral covers of the adjacent blades are in contact with each other. This blade connecting structure has high reliability owing to the high strength, which withstands centrifugal force, of the integral covers and the high vibration damping effect of friction between the adjacent integral covers.
When a blade connecting structure including integral covers is applied to turbine rotor blades having a short blade length, it is possible that the adjacent integral covers are separated from each other because the blade profile parts are twisted slightly by centrifugal force that acts on the blades during operation and thermal expansion. Therefore, in a blade connecting structure disclosed in JP-A No. 5-98906 (Patent document 1), the end surfaces, facing in the rotating direction, of the integral covers are inclined to the axis of the turbine, and the integral covers are formed in a circumferential length greater than a length obtained by dividing the circumference of a circle passing the radial positions of the integral covers by the number of blades (hereinafter referred to as “geometrical length”). Thus the integral covers of the adjacent blades are connected firmly by reaction force acting on the blades assembled so as to press each other.
Some turbine rotor blade is attached to a rotor disk of a turbine rotor by pressing the turbine rotor blade in an axial disk groove formed in the turbine disk. When the foregoing blade connecting structure is applied to the turbine rotor blades of this type, integral covers attached to the turbine rotor blades interfere with each other and the turbine rotor cannot be assembled because the length of the integral covers are greater than the geometric length. Generally, the turbine rotor blades are bent so that the integral covers may not interfere with each other when the turbine rotor blades are attached to the rotor disk. Consequently, it is very difficult to assemble the turbine rotor and a high stress is induced in the root part of the turbine rotor blade and the edge of the disk groove of the rotor disk by reaction force acting on the root part of the turbine rotor blade when the turbine rotor blade is attached to the rotor disk. After a turbine rotor has been assembled, the turbine rotor blades attached to the rotor disk twist and reaction force acts on the root parts of the turbine rotor blades. A high stress induced in the root part of the turbine rotor blade that retains the turbine rotor blade on the rotor disk against centrifugal force that acts on the turbine rotor blade during operation and edges of the disk groove in engagement with the root part of the turbine rotor blade will cause a problem in the strength of the turbine rotor that rotates at a high rotating speed.